This invention relates generally to pressure relief valves and more particularly to a repairable pressure relief valve made for simplified installation and removal.
Pressure relief valves are typically used as safety devices to ensure that the pressure within a vessel, (i.e. tank, pipe, etc.) does not exceed a safe level. A typical pressure relief valve has a spring which applies a calibrated force to a valve plug. When the pressure in a vessel (and on the plug) applies a force that exceeds the spring force on the plug, the plug will lift from its seat and vent the excess pressure. When the force on the plug is reduced below the spring force, the valve plug is pulled back into its seat by the spring force and the vessel is resealed. Without a pressure relief valve, a tank or pipe could burst, causing serious problems.
The most common failure mode of a pressure relief valve is unwanted leakage around the sealing surface of the pressure relief valve. A few pressure relief valves utilize a metal to metal sealing surface; however, this type of seal requires precision machining and alignment to create an adequate seal. Conventional pressure relief valves utilize an elastomeric (rubber) seal does not require precision machining or alignment to adequately seal; however, elastomeric materials degrade over time and lose their sealing qualities. Maintaining a good seal on a pressure vessel is critical to the proper operation of a process. In process plants where minimal leakage is critical, relief valves are high maintenance items requiring frequent inspection, replacement or repair.
Conventional pressure relief valves are either un-repairable and disposable or require factory repair. If a repairable pressure relief valve requires significant disassembly, then re-calibration is required prior to the valve""s re-entry into service. Many repairable pressure relief valves are sent to the factory for repair because field calibration is impractical. Although the replacement seal is often inexpensive, the labor, shipping, and loss of use, make factory repairable relief valves less than desirable.
Field repairable pressure relief valves are more economical, but the repair must be simple and the procedure cannot require complex tools and critical calibration. If the set point of a relief valve is unintentionally changed by a service person, system safety is compromised. More importantly, if it is possible for someone to alter a set point of a relief valve without realizing it and a disaster occurs because of the alteration, the manufacturer of the pressure relief valve could be exposed to liability.
Another important provision for repair of a pressure relief valve is ease of removal of the valve from a vessel. Most relief valves are threaded into the vessel. Often these vessels contain corrosive liquids and the threads on the relief valve bond to the threads of the vessel. As a result, the repairman must apply an extraordinary amount of force to the valve to xe2x80x9cbreak it loosexe2x80x9d from the corrosive bond to remove it from the vessel.
Typically, relief valves are flush mounted to vessels, such that the body of the valve is inside of the vessel. To remove a conventional flush mounted relief valve, a slot on the exposed face of the valve is provided for insertion of a bar (i.e. a giant flat blade screwdriver). A bar and slot configuration limits the amount of torque which can be placed on the relief valve during insertion and extraction. In fact, the slot provides only a small surface area for interfacing the bar.
Often, after a single removal of the relief valve the slot is disfigured and in subsequent installations the amount of torque that can be applied is greatly diminished. This is particularly important because the thread seal between the valve and the vessel is dependent on the amount of torque that can be placed on the valve. If a repairman cannot place enough rotational force on the relief valve, he will often resort to a hammer and chisel to pound on the slots to rotate the relief valve. Pounding on the relief valve can damage the threads of the vessel and also damage the relief valve.
A repairable, pressure relief valve having a valve body that forms an internal fluted surface is provided. The internal fluted surface is adapted to mate with a removal tool made from common, hex bar-stock. A valve assembly is slidably mounted in the valve housing and slides under excess pressure to vent fluid or gas from a vessel, such as a tank. The valve body has a first seating surface for mating with a second seating surface on the valve assembly when the valve plug is in the closed position. The relief valve has a seal for sealing an interface between the valve plug and the body and a removable cap mounted proximate to the valve plug for mechanically supporting the seal. The cap can be removed and the seal can be replaced without extensive disassembly of the valve, thereby eliminating the requirement of re-calibration after a repair has been made.